As the microelectronics industry evolves, the use of smaller microelectronic structures such as semiconductors and memory chips becomes increasingly important. Reduced circuit element size not only allows more transistors to be packed into a smaller volume but also reduces manufacturing costs.
Photolithographic processes used in microelectronics manufacturing define the size of all circuit elements. To make a semiconductor device, for instance, a silicon wafer is coated with a film of a photosensitive material and exposed through a stencil called a "mask." Radiation exposure alters the film's chemical structure in regions exposed to light. This chemical change produces differences in the dissolution rate in appropriate solvents. Selective dissolution of either the exposed or the unexposed areas of the film produces positive or negative tone images, respectively. The process produces a three-dimensional relief image in the thin polymeric film that faithfully replicates the opaque and transparent areas of the mask. This polymeric relief image is subsequently transferred to the underlying substrate by a sequence of etching, deposition or implantation steps to ultimately produce the active device. Suitable photosensitive materials must be sufficiently strong to resist the etching steps.
An important, yet unrealized, goal of the microelectronics industry has been to develop microlithography processes which operate at a wavelength of 193 nm. Microlithography processes operating at 193 nm would allow image features well below 0.25 microns to be printed. Until now, photosensitive materials such as Novolac or poly(hydroxystyrene)--the current standard materials have been useful only at wavelengths above 300 nm and above 240 nm, respectively, and cannot be used at 193 nm due to the presence of aromatic (benzene) rings. Early development of 193 nm resists was based on polymers such at poly(t-butyl methacrylate) or poly(adamantyl methacrylate) or other acrylic polymers that are optically clear at 193 nm. However, these polymers have not been sufficiently durable to withstand the harsh plasma etch steps required to transfer the resist pattern onto underlying substrates.
U.S. Pat. No. 4,491,628 discloses resist compositions made from styrene polymers in combination with a photoinitiator that releases an acid-catalyst that facilitates the chemical changes in the photosensitive material, thereby reducing the amount of ultraviolet light needed to alter the film's chemical structure. These resist materials cannot form images at 193 nm.
Norbornene polymers have been suggested for resist applications. For example, J. V. Crivello and S. Y. Shim (Chemistry of Materials, 8, 376-381 (1996)) have described electron beam resists based on polymers and copolymers of 2-t-butoxycarbonyl nobornene and 2,3-di-t-butoxycarbonyl norbornene with 4-t-butoxy-alpha-methyl styrene. The polymers are described as not possessing good oxygen plasma etch resistance. The polymers, prepared by free-radical polymerization, do not undergo cyclopolymerization. Also, the polymers are ill-defined and have linkages at the 7-position of the initial norbornene monomer as well as one of the carbon atoms that initially belonged to the carbon-carbon double bond.
Similarly, Goodall et al (PCT Application. WO 95/14048) has obtained structurally ill-controlled functionalized poly (norbornene) with both 2,3 and 2,7 linkages with a process that involves metal ion catalyzed polymerization. The metal ion catalyzed polymerization involves metal catalysts that leave metal residues in the polymer after preparation that would be detrimental to resist performance.
T. I. Wallow et al (SPIE Proceedings, 1996, 2724, 355-364) have repeated the use of a copolymer of unfunctionalized norbornene and maleic anhydride for 193 nm photolithography. Unfortunately, the copolymer has poor development characteristics. A structurally ill-defined terpolymer with 15 to 17.5% acrylic acid displays better properties, but the acrylic acid moieties decrease the etch resistance of the terpolymer even further and do not contribute to the terpolymer's imaging characteristics.
Although polymers with nortricyclene structures have been known, their preparation has often been cumbersome and resulted in ill-defined irregular structures with several different repeat units in the polymer chains. No regular uncrosslinked nortricyclene polymers have been prepared. Graham et al, J. Org. Chem., 1961, 26, 4558, describes the free radical polymerization of 2-carbethoxy-bicyclo[2.2.1]-2-5-heptadiene to afford a product that has an irregular structure with more than one repeating nortricyclene type unit as well as some 1,2-polymerized norbornene type units. Graham emphasizes its inability to achieve high conversion without gelation of the polymer as a result of 1,2-directed polymerization, thus leading to the formation of its unsaturated, crosslinked, irregular polymers. Graham contains no suggestion that regular uncrosslinked poly(nortricyclene) polymers or copolymers can be made. Graham does not utilize any bulky ester or other similar groups which have been found essential for the formation of the regular nortricyclene structures of the present invention. Also, none of the prior art nortricyclene polymers and copolymers have ever been used in any practical application, i.e. as photoresists.
Accordingly, there has been a long felt need for 193 nm microlithographic techniques that utilize imaging materials that are transparent to ultraviolet radiation at 193 nm and have good etch resistance. The present invention meets this need.
It is an object of this invention to produce regular alicyclic nortricyclene polymers and copolymers from an alicyclic norbornadiene monomer.
It is a further object to develop a resist composition from a soluble alicyclic nortricyclene polymer or copolymer.
It is a further object to overcome the polymerization difficulties of the prior art by designing a variety of bulky 2-alkoxycarbonyl norbornadiene monomers and then achieving high conversions during polymerization thereof while also avoiding contamination by metal ions.
These and still further objects will be apparent from the following description of this invention.